Former world heavyweight champion David Haye knocked fellow Briton Dereck Chisora to the canvas twice in the fifth round to win their grudge match in pouring rain at Upton Park in east London on Saturday.
The pair were involved in an ugly brawl at a news conference in Munich last February following Chisora's defeat by WBC heavyweight champion Vitali Klitschko.
Haye had flown to Germany in pursuit of a bout with Klitschko after losing to his brother Vladimir last year and subsequently announcing his retirement.
Saturday's fight at the home of English Premier League football club West Ham United for the WBO and WBA international heavyweight titles had been sanctioned by the Luxembourg Boxing Federation. The British Boxing Board of Control would not sanction the fight because neither boxer held a British licence.
The pair traded blows in a lively opening four rounds with Haye shading Chisora on points after landing a ferocious uppercut in the fourth.
In the fifth round, the former world cruiserweight champion unleashed a devastating left-right combination in the space of 10 seconds to win by a knockout.
Haye told talkSPORT radio he had been surprised by Chisora's durability.
"At the back of my mind I knew he was tough, I knew he was durable. I really trained hard," he said.
"I knew my conditioning was great and I knew the further this fight went on the more I would relax."